Once a year, a grassy stretch of cow pasture near Duncans Mills, in rural Sonoma County, loses its cows, adds some bleachers and bridges and other structures, brings in cannons and horses and piles of artillery, and then invites more than a thousand high-spirited Civil War reenactors to move in, set up camp, and play war for the weekend.

Oh right. Then they open the gates for people to come watch.

Civil War Days, considered one of the largest Civil War reenactments west of the Mississippi, loudly lets its Union-and-Confederate flags fly once again this weekend, for the 17th consecutive time at Duncans Mills. The annual event is presented by the California Historical Artillery Society (CHAS) and the Casini Ranch Family Campground. To prepare for the influx of people, and ready the battleground for two big skirmishes a day, vast teams of volunteers have been working on the site for weeks.

“It takes a lot of planning,” acknowledges Teri Moretti, a Petaluman for 50 years, and a member of CHAS since 2004. “There’s a lot of brush cutting and field clearing to be done. And then there are all of the cow patties to deal with - but we don’t pick those up. We just pay attention to where they are, and try to avoid them all weekend long.”

CHAS not only sponsors and organizes the local Civil War Days event. The group also travels around the state, participating in other Civil War reenactments.

“We go to events from Shasta County all the way down the state,” affirms Moretti. “CHAS started out as an all-male infantry unit,” she says. “It’s expanded since then, to represent all aspects of the Civil War, incorporating women and children, soldiers and civilians, artificers - all of it.”

Artificers? For those unfamiliar with Civil War military jargon, Moretti adds a succinct explanation of what an “artificer” actually does.

“Everything we break,” she says, “the artificers fix.”

For those who participate in Civil War reenactment, it’s often much more than a hobby. According to Moretti, it can become a way of life.

“Whole families participate,” she says. “That’s why CHAS can represent so many different aspects of Civil War life. Most other units in the state don’t have that flexibility. They tend to be just military.”

CHAS is a relatively large group, too.

“We have members from all over California and Nevada and Idaho,” Moretti says.

CHAS represents the Union side of the great historical conflict between the states, while some reenactment units portray the Confederacy.

“It’s usually a pretty even balance between Confederate and Union,” she says, noting that the battles staged during the weekend – two per day, morning and afternoon – are organized to allow each side an equal number of wins and losses.

“The battles are planned ahead of time,” Moretti explains. “The North wins one on Saturday and one on Sunday, and the South wins one on Saturday and one on Sunday. Everybody gets an equal shot.”

Civil War Days is more than just a demonstration of North vs. South battle tactics. Before and after the skirmishes, visitors are invited to walk about through the military camps, and the adjacent “sutler” camp – sutlers being costumed merchants selling a variety of Civil War-themed items, from period garb and other gear to books, toys, and handcrafts. Visitors to the reenactor camps are encouraged to ask questions, take pictures, or just stand back and observe the authentic living history that participants work so hard to maintain.

WHAT? Civil War Days, a reenactment of life and warfare during the American Civil War.

Some visitors find the whole thing so intriguing they’re ready to sign up then and there. That’s not necessarily out of the question. The event’s website has detailed information about what it takes to become an active participant.

“You do have to be a member of a particular unit,” Moretti allows, explaining that most units keep a “loaner box” of authentic uniforms and other period garb.

“So if somebody wants to join up on the spot,” she says, “they can.”

They’ll have to sign a sheaf of legal waivers, and take a quick training in proper battleground rules and regulations, of course. They’ll even have to pass a safety test. Most newbies do little more than advance and retreat, leaving the more complicated details – shooting off cannons, riding of horses, hand-to-hand combat – to those with proven experience.

On occasion, Moretti says, someone will show up in their own Civil War uniform and volunteer to join the unit. But even their costume will have to pass a rigorous test.

“Authenticity is pretty high,” she says. “No polyester, no Halloween costumes. Some groups are more restrictive than others.”

Among Moretti’s many duties with the organization is securing the paperwork and legal agreements required for staging so elaborate and (literally) explosive an event.

“I start working on the permit process in January,” she says, adding that as the sponsoring organization, CHAS does more than just clear the site and build the bleachers and make sure everything is legal and safe. “We also provide water, the restroom facilities, and all the wood the various units use for campfires.”

Moretti is looking forward to this year’s Civil War Days, having had to miss the actual event for the last four years due to a job that kept her busy on weekends.

“Normally I am one of the horse drivers,” she says. “This year, I’m the provisioner for my unit. My husband has been the provisioner for years, but this year, he has to work. So I’m the provisioner.”

And what, exactly, is a provisioner?

“A provisioner feeds the unit,” she says. “We plan the menu, cook the food, and cleanup afterward. And let me tell you, after eating camp food for two or three days, on Sunday night, when it’s all over? We go out to a restaurant. It’s a pretty good way to celebrate finishing another weekend of bringing history to life.”